The present invention relates to a heat-sensitive transfer medium for use with a thermal transfer printer to be utilized as an output device for a word processor, computer, facsimile, etc., and more particularly to a heat-sensitive transfer medium capable of ensuring reliably good printing quality.
FIG. 2 is a schematic side view of an essential part of a thermal transfer printer in the prior art. Reference numeral 1 designates the cross-section of a platen extending longitudinally from one side of the printer to the other. A platen rubber 2 is attached on a front surface of the platen 1. A columnar support shaft 3 is disposed in front of and below the platen 1 and extends in a longitudinal direction parallel to platen 1. A carriage 4 is supported on and is movable along the support shaft 3 in the longitudinal direction parallel to platen 1. A ribbon cassette 5 containing an ink ribbon (as an example of a thermal transfer medium) is removably mounted on the carriage 4. A head mounting base 7 is rotatably supported on the support shaft 3. A thermal head 8 is supported on the head mounting base 7 which is movable together with the carriage 4 in the longitudinal direction along support shaft 3 parallel to platen 1. A guide plate 9 projects rearwardly from the head mounting base 7. The guide plate 9 is operatively engaged with a U-shaped portion 12 formed at one end of a rocking lever 11 rotatably supported on a pin 10. The rocking lever 11 is connected at its other end to a tension spring 13 for normally biasing the rocking lever 11 in a clockwise direction (as viewed in FIG. 2). Accordingly, the head mounting base 7 is normally biased by the tension spring 13 in a counterclockwise direction (as viewed in FIG. 2) via the rocking lever 11 and the guide plate 9. As a result, the thermal head 8 is normally pressed against the platen 1. A solenoid 14 is operatively connected to the rocking lever 11 in such a manner that when the solenoid 14 is energized, the rocking lever 11 is rotated in a counterclockwise direction, as viewed in FIG. 2, against the biasing force of the tension spring 13, thereby rotating the head mounting base 7 in a clockwise direction and separating the thermal head 8 and the platen 1.
FIG. 3 is a plan view of the ribbon cassette 5. The ink ribbon 6 contained in the ribbon cassette 5 is partially exposed between a ribbon outlet 5A and a ribbon inlet 5B. The ink ribbon 6 is collected on a rotating drum 15 by rotating the rotating drum 15 in a direction as depicted by the arrow shown in FIG. 3. Accordingly, the ink ribbon 6 is pulled out of the ribbon outlet 5A, and is drawn into the ribbon inlet 5B. The ribbon cassette 5 is formed with a recess 16 at a position behind the exposed portion of the ink ribbon 6. The head mounting base 7 and the thermal head 8, when the ribbon cassette 5 is mounted on the carriage 4, are disposed in the recess 16 behind the exposed portion of the ink ribbon 6.
Referring to FIGS. 4 and 5 which show a retracted positioned and an advanced position of the thermal head 8, respectively, a pair of wires 17 are connected to opposite ends of the carriage 4 to move the carriage 4 longitudinally along the support shaft 3. The carriage 4 is formed with a recess 18 for receiving the head mounting base 7. The head mounting base 7 is moved in the recess 18 such that the thermal head 8 comes into contact with and is separated from the platen 1 by the operation of the tension spring 13 and the solenoid 14.
When the solenoid 14 is deenergized, the rocking lever 11 is rotated in the clockwise direction, as viewed in FIG. 2, by the biasing force of the tension spring 13 which causes the head mounting base 7 to rotate in the counterclockwise direction. As a result, the head mounting base 7 is moved in the recess 18 from the retracted position shown in FIG. 4 to the advanced position shown in FIG. 5. In the advanced position of the head mounting base 7, the ink ribbon 6 and a printing paper 19 (see FIG. 2) are sandwiched between the thermal head 8 and the platen 1. The heating elements located in the thermal head 8 are then energized in a predetermined pattern, which depends upon, e.g., the character to be printed. As a result, the ink ribbon 6 is heated by the thermal head 8, causing the ink to be melted in the predetermined pattern. Because the ink layer 21 is pressed against the paper when it is melted, the ink is attracted to and is therefore transferred onto the paper 19, thus resulting in the transfer of, for example, a printed character from the ink ribbon 6 to paper 19.
FIG. 6 is an enlarged side view of a section of the ink ribbon 6 used as the heat-sensitive transfer medium in the prior art. As shown in FIG. 6, the ink ribbon 6 is formed by laminating an ink layer 21 onto a base layer 20. The base layer 20 is comprised of a plastic film such as polyethylene terephthalate (PETP), and the ink layer 21 is comprised of a binder primarily containing a wax such as paraffin wax and carnauba wax or a low-molecular resin such as ethylene vinylacetate copolymer (EVA) and polyamide, a coloring agent such as pigment (e.g., carbon black) and dyestuff (e.g., oil black), a softening agent such as silicone oil, a dispersing agent such as stearic acid, and antiseptics.
In recent years, there has been a demand for printers which can produce printing with reliably good quality even on rough paper, i.e., paper having a large surfaceroughness. The prior art solution to this demand is to increase the amount of resin mixed with the ink, which causes the ink to have a greater affinity for the paper. However, the resulting increase in the affinity of the ink for the paper also causes an increase in the adhesive strength between the ink layer 21 and the base layer 20. Accordingly, if the amount of resin in the ink is significantly increased, the ink layer 21 will fail to separate properly from the base layer 20 during printing. Thus, this prior art solution to the demand for reliably good printing quality on rough paper is inadequate.
To improve the quality of printing incorporating resin to increase the affinity of the ink to rough papers, there has been proposed a second solution in the form of the ink ribbon 6A shown in FIG. 7. In ink ribbon 6A an ink releasing layer 22 is interposed between the base layer 20 and the ink layer 21. The ink releasing layer 22 is composed of a wax such as amide wax (e.g., fatty acid amide) and a softening agent such as silicon oil. The wax is produced to have a melting point or a softening point not lower than that of the ink layer 21 and to have a molten viscosity lower than that of the ink layer 21.
When printing with the ink ribbon 6A, the ink releasing layer 22 is melted along with the ink layer 21 by the heat from the thermal head 8. When the ink releasing layer 22 is melted, the cohesion within the ink releasing layer becomes weak, thus allowing the ink layer 21 to be easily separated from the base layer 20 and thereby contributing to the transfer of melted ink from the ink layer 21 to the paper 19. When the ink releasing layer 22 is cooled and solidified, the cohesive strength within the wax is high and the wax adheres to both the base layer 20 and the ink layer 21. Thus, when the wax is cooled and solidified, the ink is not easily separated from the base layer 20. Accordingly, even when the amount of resin mixed in the ink layer 21 is increased, the melted ink can be smoothly transferred from the ink layer 21 to the paper 19 owing to the presence of the molten ink releasing layer 22, thereby ensuring good printing quality even on the rough paper.
However, when the amount of resin mixed in the ink is increased, it also causes delayed separation of the ink ribbon 6A from the paper 19 due to the increased adhesion between the ink and the paper. This delayed separation causes the heat (transmitted from the thermal head 8) of the molten releasing layer 22 to be dissipated too quickly and the ink releasing layer 22 to become solidified before the ink ribbon has separated from the paper. As a result, the ink releasing layer 22 cannot effectively contribute to the release of the molten ink from the ink layer 21 because the molten ink has a greater attraction for the solidified ink releasing layer 22 than for the paper 19. Therefore, the ink of the ink layer 21 cannot be reliably transferred onto the printing paper 19. That is, the ink, once having been transferred onto the printing paper 19, is transferred back to the ink ribbon 6A (which is called "inverse transfer") by the attraction between the ink and the hardened ink releasing layer 22. Inverse transfer causes deterioration of printing quality in the form of characters with missing parts or in chipping of the ink making up the printed characters. Also, the attraction of the ink to both the paper 19 and the base layer 22 causes the removal of the surface of the paper 19 when the ink is pulled from the paper by the ribbon 6A. Thus, the printing quality is greatly deteriorated.